Propagation of star formation at sub-kiloparsec scales

Abstract

We study the propagation of star formation based on the investigation of the separation of young star clusters from H ii regions nearest to them. The relation between the separation and U − B colour index (or age) of a star cluster was found. The average age of star clusters increases with the separation as the 1.0-1.2 power in the separation range from 40 to 200 pc and as the 0.4-0.9 power in the range of 100-500 pc in the galaxies with symmetric morphology. The galaxies with distorted asymmetric disc structure show more complex and steeper (power > 1.2 at separations from 40 to 500 pc) dependence between the age and the separation. Our results confirm the findings of previous studies on the dominant role of turbulence in propagation of the star formation process on spatial scales up to 500 pc and on time scales up to 300 Myr. On a smaller scale ( 100 pc), other physical processes, such as stellar winds and supernova explosions, play an important role along with turbulence. On the scale of stellar associations (100-200 pc and smaller), the velocity of star formation propagation is almost constant and it has a typical value of a few km s −1 .

Azimuthal propagation of star formation in nearby spiral galaxies: NGC 628, NGC 3726 and NGC 6946

“…Unlike star formation induced by a stationary spiral shock, a stochastic selfpropagating star wave has no dedicated propagation direction and the age gradient caused by stochastic star formation makes it difficult to detect the azimuthal age gradient caused by a spiral shock. Gusev & Shimanovskaya (2019) showed that, contrary to the spiral structure, trends emerge in the galaxies they studied, suggesting that in some cases spiral density waves are not the dominant mechanism. On the other hand, the global spiral patterns in Grand Design galaxies, which is the case of NGC 628 studied here, certainly results from a spiral density wave.…”

“…To find a systematic variation in the azimuthal offset in SC-HiiR pairs as a function of galactocentric distance, we used the coordinate pairs presented in Gusev & Shimanovskaya (2019). We selected three galaxies NGC 628, NGC 3726 and NGC 6946 with a representative sample of SC-HiiR pairs (see Table 1) from the sample of five nearby galaxies studied in Gusev & Shimanovskaya (2019). Photometric U BVRI and Hα+[Nii] observations taken at the 1.5-m telescope of Mt.…”

“…The plate coordinates of all objects (star clusters in B images and H ii regions in Hα images) were recalculated in deprojected ones, using corresponding position angles and inclinations of galaxies. As objects of study, we used the "stellar cluster-H ii region" pairs selected earlier in Gusev & Shimanovskaya (2019). To avoid random SC-HiiR pairs, the age of clusters among the candidates was controlled using the relationship between the U − B colour index and the cluster age predicted by stellar population evolution models.…”

“…The velocity of a stochastic self-propagating star-formation wave determines the magnitude of the spatial spread of stars of different ages in the regions of star formation, the so-called age gradient. Gusev & Shimanovskaya (2019) found a relation between the spatial separation S of young star clusters from the nearest H ii regions and the age t (U − B colour index) of star clusters on spatial scales from 40 to 500 pc and on time scales from 10 to 300 Myr in five spiral galaxies: NGC 628, NGC 3184, NGC 3726, NGC 5585, NGC 6946. Unlike star formation induced by a stationary spiral shock, a stochastic selfpropagating star wave has no dedicated propagation direction and the age gradient caused by stochastic star formation makes it difficult to detect the azimuthal age gradient caused by a spiral shock.…”

“…The contribution of the spiral shock wave to star formation should manifest itself through a characteristic change in the azimuthal component of the age gradient as a function of the distance from the galactic centre, or the azimuthal component of the spatial displacement of young star clusters with nearby H ii regions. Therefore, the measurements of Gusev & Shimanovskaya (2019) can also be used to study star formation induced by a spiral shock wave.…”